Shining light on integrity of a tetracobalt-polyoxometalate water oxidation catalyst by X-ray spectroscopy before and after catalysis

Abstract

Modification of the Co-oxo cores of cobalt-polyoxometalate water oxidation catalysts is detectable by X-ray absorption spectroscopy (XAS) as demonstrated by comparison of Na10[Co4(H2O)2(PW9O34)2] (1) and Na17[((Co(H2O))Co2PW9O34)2(PW6O26)] (2). XAS reveals the integrity of 1 uncompromised by oxidant-driven water oxidation, which proceeds without formation of catalytic cobalt oxide.

Solution Speciation and Stability of Cobalt‐Polyoxometalate Water Oxidation Catalysts by X‐ray Scattering

“…According to these results and previously reported studies on the degradation of Co 4 in NaP i buffered solutions, [19,23] it seems obvious that the presence of high concentration of NaP i buffer may accelerate the degradation of Co 9 and Co 4 : precipitation of cobalt oxide phosphate will shift the equilibrium towards release of more cobalt into solution. However, this effect has not yet been investigated in detail.…”

“…[19,23] This stability depends on the phosphate concentration, for experiments using 0.05 m NaP i buffer (Figure S14b and Table 3), the drop in the scattered intensity is reduced in comparison with the experiments using 0.9 m NaP i (Figure 6), representing an intermediate between a fully-buffered and a completely non-buffered system. Our results suggest that in the presence of phosphate, these free Co 2+ ions rapidly form a precipitate.…”

“…[15][16][17][18][19][20][21][22][23] Following its initial report, [5] investigations under electrochemical conditions indicated that Co 4 decomposes to generate a CoO x film that becomes the main catalyst. [15][16][17][18][19][20][21][22][23] Following its initial report, [5] investigations under electrochemical conditions indicated that Co 4 decomposes to generate a CoO x film that becomes the main catalyst.…”

“…[4][5][6][7][8][9][10][11][12][13] One of the most noteworthy advantages of the use of POM ligands for the design of homogeneous WOCs is that they are entirely inorganic, eliminating the problem of oxidative degradation of organic ligands. [15][16][17][18][19][20][21][22][23] The main issue concerning the catalytic activity of Co 4 and related catalysts resides in the fact that, despite their redox stability, cobalt ions are released into water, which are in equilibrium with the POM. Our studies reveal that: 1) the clusters remain largely intact during reaction with a chemical oxidant, especially in the absence of phosphate buffer; and 2) common phosphate buffers have the greatest effect on the degradation of the Co-POMs under these conditions.…”

“…Our studies reveal that: 1) the clusters remain largely intact during reaction with a chemical oxidant, especially in the absence of phosphate buffer; and 2) common phosphate buffers have the greatest effect on the degradation of the Co-POMs under these conditions. [17][18][19][26][27][28][29][30] Within the plethora of recent studies on cobalt tungstate POMs as WOCs, the focus has been predominantly on 1) the release of Co 2+ ions into solution; and 2) the formation of CoO x on the electrode or as nanoparticles, and the effect of these interrelated phenomena on catalytic activity. [5] this particular catalyst has been thoroughly investigated and debated since its initial report.…”